Such a power transmission device for an astride-riding vehicle is already known as disclosed in, for example, Japanese Patent Application Laid-open No. 2001-105933.
Conventionally, with regard to the characteristics of a torque converter for a general vehicle, in order to minimize creep while idling, as shown in FIG. 10, a pump capacity is set to be a maximum at a speed ratio e of about 0.5 (high speed ratio), and the pump capacity at a speed ratio e of 0 is set to be slightly lower than the maximum pump capacity. The pump capacity is also set to gradually decrease when the speed ratio e exceeds about 0.5, to rapidly decrease when the speed ratio e exceeds 0.8 (coupling point), and to eventually be a minimum. Setting the pump capacity in this way eliminates the sensation of slip when cruising.
As described above, since the pump capacity changes only a little when the speed ratio e changes from 0 to 0.5, as is clear from FIG. 11, the gap between the pump torque curve at a speed ratio e=0 and the pump torque curve at a speed ratio e=0.5 is small. As a result, as shown by the broken line in FIG. 9, even when the speed ratio e increases from 0 to 0.5, the engine rotational speed hardly increases because of the balance between the engine torque and the pump torque. Therefore, when an acceleration operation of a throttle valve is carried out from an idling state, the turbine rotational speed, in other words, the vehicle speed, increases without a large increase in the engine rotational speed.
However, in order to achieve good drivability, sport astride-riding vehicles require linear properties in which both the engine rotational speed and the turbine rotational speed increase substantially linearly in response to an acceleration operation of the throttle valve.